The present invention relates to the preparation of aqueous, solvent-free or low-solvent, storage-stable PU crosslinker dispersions having pyrazole-blocked isocyanate groups, to coating compositions prepared from them and to their use in coatings.
Recent years have seen a sharp rise in the profile of aqueous paints and coating compositions in the wake of increasingly stringent emissions directives governing the solvents released during paint application. Although for many fields of application there are now aqueous paint systems already available, these systems are often unable to attain the high quality level of conventional, solvent-borne paints with respect to solvent resistance and chemical resistance, elasticity and mechanical durability.
Even water-borne paint systems, based on aqueous polyurethane dispersions, frequently still contain considerable amounts of solvents. The complete elimination of these solvents is generally not possible for polyurethane dispersions, since the preparation of such dispersions via prepolymers often necessitates solvents, or it is often necessary to add what is called a cosolvent (coalescence agent) to the dispersions in order to achieve a reduction in the minimum film-forming temperature. The cosolvent ensures that when a film is formed of the coating compositions, even at or below room temperature, coats are formed which are sufficiently hard. In addition, in the absence of solvents, the polyurethane dispersions and the paint formulations frequently lack storage stability.
The solvent N-methylpyrrolidone (NMP) in particular is in widespread use in the field of aqueous dispersions and paints. For example, the carboxylic acid-hydrophilicized polyisocyanate crosslinker dispersions with dimethylpyrazole-blocked isocyanate groups described in EP-A 0942023 contain NMP as a cosolvent.
Cosolvent-free preparation of the dimethylpyrazole-blocked polyisocyanate crosslinker described in EP-A 0942023, by elimination of the solvent, is not possible, due to the increase in viscosity.
The nonionically hydrophilicized polyisocyanate crosslinker dispersions with pyrazolically blocked isocyanate groups that WO 1997012924 describes contain approximately 7% butyl glycol as cosolvent. Butyl glycol, similarly to NMP, is distinguished by a relatively high boiling point. Its removal in order to prepare solvent-free dispersions is not possible.
DE 19 914 885 describes polyurethane dispersions having dimethylpyrazole-blocked isocyanate groups for preparing glass fibre sizes. These dispersions are prepared using an organic solvent, which after dispersion in water, is removed from the dispersion by distillation. Example 1 describes the preparation in a 62 percent strength by weight acetonic solution. These dispersions are constructed using polyol components having molecular weights of at least 350 g/mol in a fraction of 30% to 90% of the prepolymer. As a result of the high fraction of soft segments, the dispersion as described cannot be used as a crosslinker component for high-value coating compositions, since the paints are unable to attain sufficient hardness es.
DE 3 613 492 describes an acetone process for preparing cosolvent-free polyurethane-polyurea dispersions. The prepolymer, which in this case is not blocked, is prepared in a 20 to 50 percent strength by weight solution in a volatile organic solvent, such as acetone, and following dispersion in water, the solvent is removed by distillation.
The replacement of NMP in the process of EP-A 0942023 by acetone in amounts of 50 or 62% by weight, as in DE 19914885, leads to dimethylpyrazole-blocked polyisocyanate crosslinkers which, are not storage-stable.
The object of the present invention, then, is to provide storage-stable polyurethane dispersions having pyrazole-blocked isocyanate groups in a similar way to that in which they can be obtained in accordance with EP-A 0 942 023, but which are solvent-free or low in solvent and in particular do not contain NMP or butyl glycol. Furthermore, the coatings produced from these dispersions should have good film optical properties, good chemical resistance and pendulum hardnesses of more than 80 seconds.